Skepticism is a very ancient concept. In
philosophy, it is the doctrine that absolute knowledge is not
achievable. Therefore, inquiry must proceed by doubt and
acquisition of approximate or relative knowledge. This is pretty
much the way the modern scientific method works, even though many
scientists would not consider themselves "skeptics" in
a philosophical sense. Interestingly, one of the first modern
books on the scientific method was entitled A skeptical
chemist, and was authored by the father of modern chemistry,
the Anglo-Irish physicist Robert Boyle
(1627-1691).

The very first skeptics were perhaps the Greek
sophists of the 5th century BC. They were philosophers of
pre-Socratic time, inquiring about theology, metaphysics, natural
sciences and mathematics. They specialized in dialectic and
rhetoric, from which they got their bad reputation as specious
thinkers. In fact, sophists believed that one should exercise
his/her mind by taking different sides of the same argument and
attempting to defending all with equal effectiveness. They are
probably the precursors of modern lawyers...

Protagoras (490-421 BC) was the main skeptic of
ancient times. He was a Greek philosopher, leader of the sophist
movement. His most famous statements is perhaps "Man is the
measure of all things". By that he meant that truth is
relative to the individual who maintains it, i.e., that there are
no universal truths. He denied the possibility of objective
knowledge, and went so far as to deny a difference between reason
and sense. No work by Protagoras has actually reached us
directly, but his ideas are presented in some of Plato's
dialogues.

The first "modern" skeptic was David Hume
(1711-1776), a Scottish philosopher. He elaborated on the
philosophy of two other great thinkers: the empiricists John Locke
(1632-1704), and George Berkeley (1685-1753). Let us take a quick
look at these two, before going back to Hume. The English Locke
opposed the existence of innate ideas, suggesting instead that
the human mind is born as a blank board (tabula rasa) on
which experiences inscribes the character. According to Locke,
science is possible because the primary qualities of objects in
the world affect human sense organs mechanically. The Irish
Berkeley pushed Locke's ideas a step further. His subjective
idealism maintains that matter does not exist independently of
perception. We perceive a "real" world, according to
Berkeley, only because of the mind of God. Hume went beyond both
Locke and Berkeley, with his notion of radical skepticism. He
suggested that the mind only perceives sensations, and that
therefore there is no certain knowledge. A corollary was the
implication that our perception of cause and effect is just that,
a perception. As for religion, Hume rejected any form of
theology, natural or rational.

Immanuel
Kant (1724-1804) and his agnostic doctrine is probably more
close to a modern understanding of skepticism. A German
philosopher, Kant openly acknowledged having being turned into a
critical philosopher by reading Hume. He thought of his work as
of a synthesis of Hume's skepticism and Leibnitz's
rationalism (Gottfried Wilhelm, baron von Leibnitz was a German
mathematician born in 1646, who died in 1716. His philosophy was
quite naive, claiming that the existing world was the best of all
possible worlds, a view satirized by the French philosopher
Francois Marie Arouet de Voltaire
(1694-1778) in his hilarious novel Candide).
But let us get back to Kant. According to Kant, objective reality
is known only to the extent that it conforms to the structure of
our mind. He made a distinction between phenomena, which
are knowable by experience, and noumena, which lie
beyond experience and therefore cannot be demonstrated to exist
or not exist. The main consequence - discussed in his Critique
of pure reason - is that God, freedom, and similar can
simply not be investigated by scientific means.

The first author to use skepticism as a method of
inquiry was the French Rene` Descartes
(1596-1650), chiefly exposed in his Discourse
on method. Descartes idea was to extend the mathematical
approach to discovery to every realm of human endeavor. His
method therefore started out by doubting everything, and
attempting to derive everything from the only thing that could
not be doubted, that it doubt itself. If that is correct, then
the doubter must exist, hence his famous cogito ergo sum.
He then expanded gradually his inquiry, to include the existence
of God as the first cause of the universe, and the existence of
the physical universe itself.

The modern scientific method, as mentioned above,
is to some degree based on skepticism. It needs to be remembered,
however, that scientific positivism requires that material effect
is possible only with a material cause. A spin off of skepticism
is the philosophy of logical positivism, established in the 1920s
by a group of thinkers known as the Vienna circle (including
people like Kurt Godel, Bertrand Russell,
and Ludwig Wittgenstein). Logical positivists maintain that
metaphysical speculation is nonsensical, and that moral and value
statement are merely emotive. Russell,
in particular, suggested that individual facts are logically
independent, while knowledge is dependent on data from
experience.